Fire resistant, moisture barrier membrane

ABSTRACT

A waterproof and breathable, fire-resistant laminate is provided for use in tents, garments, shoes, and covers, especially in industrial, military and emergency situations. The laminate permits water vapor evaporation while simultaneously preventing liquid water penetration. Further, the laminate is fire-resistant and significantly reduces the danger of toxic compound production when exposed to flame or other high heat source. The laminate may be applied to a variety of substrates and is comprised of a silicone rubber and plurality of fire-resistant, inherently thermally-stable polyimide particles.

ORIGIN OF THE INVENTION

The invention described herein was made by a NASA employee and may bemanufactured or used by or for the Government for governmental purposeswithout the payment of any royalties thereon or therefor.

This application claims the benefit of U.S. Provisional Application Ser.No.: 60/008,765 filing date Dec. 15, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to waterproof/breathable ("WPB")fabric coatings and laminates. More specifically, the present inventionrelates to fire-resistant, WPB fabric coatings and laminates whichminimize the risk of toxic compound production when exposed to flame orother high heat source.

2. Description of the Related Art

The markets for and uses of waterproof/breathable ("WPB") fabrics havegreatly expanded since the development of Gore-Tex® and similarlaminates. The advantages of WPB fabrics is their ability simultaneouslyto repel liquid water (waterproof) and permit the passage of water vapor(breathable). In practical terms, one wearing a WPB rain garment ininclement weather, even while exercising, would stay dry because therain is kept out but perspiration evaporates away. This is a greatimprovement over fabrics which are waterproof but not breathable (e.g.polyurethane coated fabrics or polyvinyl chloride films) or breathablebut not waterproof (e.g. cotton). The application of silicone orfluorocarbon or other water repellant to a breathable fabric (e.g.cotton) might provide some minimal water repellency, but falls far shortof the performance of WPB fabrics.

Hydrophobic membranes are ideal candidates for use in WPB fabrics. Thehydrophobic membrane contains very small pores that resist the entry ofliquid water even at substantial pressures or when rubbed or flexed, butreadily allow the flow of gases like water vapor. This is to becontrasted with wicking materials. Wicking materials are hydrophilic andporous with pores that interconnect to make complete pathways throughthe material. Liquid water moves through these materials by capillaryaction. While wicking materials easily transport water and water vaporfrom an internal to an external side, they offer no resistance to theentry of liquid water. Gore describes the use of hydrophobic layers inWPB fabrics in U.S. Pat. No. 3,953,566.

An example of a membrane for a WPB fabric is detailed in Gore et al.,U.S. Pat. No. 4,194,041, where the membrane is a microporouspolytetrafluoroethylene ("PTFE") laminate that possesses a high moisturevapor transmission rate even under adverse climatic conditions. Thisinvention comprises a first layer of hydrophobic material attached to asecond layer of hydrophilic material. A waterproof/breathable fabric isconstructed by attaching this membrane to a suitable base fabric such asnylon taffeta. The hydrophobic layer prevents the entry of liquid waterinto the fabric, while the hydrophilic layer draws the interior moistureto the membrane so that it might evaporate through. Importantly, thehydrophilic layer also serves to prevent oils and contaminates inperspiration from entering the hydrophobic layer, coating its interiorsurfaces, and greatly reducing its ability to repel water. The Gore-Tex®membrane is available commercially from W. L. Gore & Associates, Inc.

Performance fabric technology incorporating WPB membranes has continuedto advance rapidly. Gohlke, U.S. Pat. No. 4,344,999, developed abreathable laminate that serves as a bacteria barrier for hospitalapplications. Worden details a process for making stretchable Gore-Tex®membranes in U.S. Pat. No. 4,443,511. An anti-static WPB fabric forcovering aerospace equipment was described by Saville et al. in U.S.Pat. No. 4,816,328. In U.S. Pat. No. 4, 868,928, Norvell described astretchable WPB garment for active outdoor gear. An improved Gore-Tex®membrane allowing for a greater water vapor transmission rate wasdescribed by Henn in U.S. Pat. No. 4,969,998. Wu, in U.S. Pat. No.5,242,747, describes the use of oleophobic material in a WPB membrane toprevent damage and clogging from oils.

Generally speaking, there are two types of performance fabrics whichexhibit WPB and windproof qualities: laminates and coated fabrics.Coated fabrics are created by applying rubber, PVC, polyurethane,ceramic or amino acid compounds to a fabric with a spray or bath orother method. In contrast, laminates are produced by sticking one ormore membranes to a fabric via heat sealing or adhesives.

The Gore-Tex® membrane is generally laminated to a high performancefabric to create a laminated fabric. Some constructions even sandwichthe Gore-Tex® membrane between two fabrics for additional comfort inwearing as personal garment. In general, these laminated fabrics areable to withstand very high water pressure (up to 65 psi) with minimalleakage, but "breathe" as water vapor passes through the fabric/membrane away from the body. This feature is due to the billions ofmicroscopic pores in the membrane which are small enough to restrictwater droplets but large enough to allow water vapor passage. One commonGore-Tex® membrane is a composite of a hydrophobic (water-hating)material into which is integrated an oleophobic (oil-hating) substance.The hydrophobic material prevents water droplets from penetrating thefabric. While allowing moisture vapor to pass through, the oleophobicmaterial prevents penetration of contaminates like oils, insectrepellents, and food.

Such a membrane is comprised of a rubbery or elastomeric binder(hydrophobic) and a polymeric filler of micron-sized particles(oleophobic). Alone, the rubbery material blocks the penetration ofwater; minuscule pathways are created by the addition of the polymerwhich allow for the passage of water vapor. The resulting membrane isideal for use in WPB fabrics.

However, a significant problem arises with these WPB laminates. Theyoften employ natural or synthetic rubbers in combination with afluorinated polymer, e.g. Teflon®. When exposed to a high heat source orflame, the rubbery portion of this combination can burn and generatetoxic by-products. The fluorocarbons also have the tendency to decomposein fire to produce toxic compounds.

Some have attempted to make WPB fabrics fire-resistant by applying aflame-retardant composition to the fabric (see U.S. Pat. No. 4,223,066).This method is deficient because of the tendency of the surface coatingsto wash off during cleaning. Sun, in U.S. Pat. No. 5,418,054, details afire-resistant WPB laminate comprised of two layers of expanded PTFEsealed together with a phosphorous-containing poly(urea-urethane)adhesive. While retaining its fire-resistant qualities even throughcleaning or laundering, this invention does not address the danger oftoxic compound production.

There is an ever present need for new, improved WPB laminates for highperformance fabrics which are fire resistant and either do not presentor significantly reduce the danger of toxic compound production. Inparticular, there is a need for such WPB laminates for fabrics to beused in industrial, military, commercial and emergency situations.Specifically, military tents and gear and shoes, airplane seat covers,clothing for fire-fighters or industrial workers exposed to high heatsources, and commercial fabrics for tents or roofs require waterproof,breatheable, fire-resistant and safety qualities. Laminates in theavailable art have been unable to provide the improved fire-resistantand safety features required by the described applications.

SUMMARY OF THE INVENTION

It is accordingly the primary object of the present invention to providewhat the prior art has been unable to provide, viz., an improvedlaminate for use in WPB, high performance fabrics which is fireresistant and significantly reduces the danger of toxic compoundproduction when exposed to flame or other high heat source. It is afurther object to provide a laminate readily applied to fabrics for usein garments, tents, covers, and shoes.

The primary object and other objects and benefits are achieved by thepresent invention, a waterproof and breathable, fire-resistant laminatecomprised of a silicon rubber and a polyimide powder, the polyimidepowder comprising a plurality of polyimide particles. The particles aresuspended in the silicone rubber so that a plurality of pathways arecreated through the laminate which allow the passage of water vapormolecules but not liquid water. The laminate is formed by applying aslurry of a silicone precursor and the polyimide powder to a fabric orother suitable material, and concurrently or sequentially setting thelaminate thermally or with a catalyst or with moisture, depending on thenature of the silicone precursor. Once the laminate has been "set," i.e.the slurry has been polymerized to a rubbery material, it will preventliquid water from passing through the fabric but will allow thetransmission of water vapor.

The silicone rubber is formed from a silicone precursor such as adimethylsilicone fluid terminated with reactive end groups, where thereactive end groups cause molecular growth to polymer form via standardchemical techniques. The dimethylsilicone fluid with reactive end groupsis represented as

    X--Si(CH.sub.3).sub.2 -- OSi(CH.sub.3).sub.2 !.sub.m --X

where m=5-1000, and X is equal to one of the group comprising a mixtureof --CH═CH₂ and --H, --OCOCH₃, or --OCH₃.

The polyimide powder comprises of a plurality of polyimide particles.The particles may be approximately round and may be of approximately thesame diameter, but need not be. Irregularly shaped and/or sizedparticles may be used. Particles of an oblong shape or an approximatelyrectangular shape, e.g. produced from chopped polyimide fibers, mayyield beneficial results. In fact, better results may be achieved withparticles exhibiting higher aspect ratios.

The polyimide powder must be of a fire-resistant, inherentlythermally-stable polyimide. It is preferable to use a fire-resistant,inherently thermally-stable, linear aromatic polyimide. Beneficialresults are achieved from using polyimide powder LARC™-IA. The chemicalstructure of LARC™-IA is illustrated below: ##STR1## where n=10-100.

The particles of the polyimide powder should be micron-sized, measuringapproximately 1-20 μm in size. The loading level of the powder should bebetween 20-60% by volume.

The polyimide powder is physically blended into the silicone precursorto form the slurry. Once the slurry has been set to form a siliconerubber membrane, the polyimide particles, suspended therein, serve tocreate chemical and physical pathways through which water vapormolecules are able to pass. As a result, the waterproof/breathablequalities are achieved.

The process for producing the laminate of the present invention is morefully described in the below section.

Beneficial results are achieved where the WPB, fire-resistant laminatecomprises a silicone rubber and a polyimide powder, the polyimide powdercomprising a plurality of fire-resistant, inherently thermally-stable,linear aromatic polyimide particles, where the polyimide is LARC-IA™.The polyimide particles used are of variable shape and size, where thesize is between 1-20 μm, and the particles have a high aspect ratio. Theloading level of the powder is approximately 30%. The silicone rubber isformed from a silicone rubber precursor, where the silicone rubberprecursor is a dimethylsilicone fluid with reactive end groupsrepresented as

    X--Si(CH.sub.3).sub.2 -- OSi(CH.sub.3).sub.2 !.sub.m --X

where m=5-1000, and X is equal to an equimolar mixture of --CH═CH₂ and--H.

A slurry is formed from the silicone rubber precursor and the polyimideparticles. To the slurry, a platinum catalyst is added to set thesilicone rubber. The slurry is soon thereafter applied to the substrate,e.g. nylon taffeta, by the process of doctoring to form a membrane ofaverage thickness between 0.0002 and 0.002 inches on the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The laminate of the present invention can be produced as describedbelow. A polyimide powder is physically blended into silicone precursor,namely a dimethyl-silicone fluid terminated in reactive end groups, toform a slurry. The slurry can be applied to a fabric, cloth or othersuitable substrate via a variety of techniques including brushing,calendering, rolling, doctoring, or silk screening. The application ofthe slurry to the cloth, or other substrate, should afford a liquidmembrane of a thickness in the range of approximately 0.0002 to 0.002inches. After application, the slurry coating is set, polymerized toform the rubbery membrane, with either moisture or heat or a catalyst.The method of setting depends on the nature of the silicone precursor.Polyimide powder, e.g. LARC™-IA, of a size approximately 1-20 μm indiameter is physically blended into the silicone precursor to a loadinglevel of 20-60% by volume.

The silicone precursor, a dimethylsilicone fluid with reactive endgroups, is represented as

    X--Si(CH.sub.3).sub.2- -- OSi(CH.sub.3).sub.2 !.sub.m --X

where m=5-1000, and X is equal to one of the group comprising a mixtureof --CH═CH₂ and --H, --OCOCH₃, or --OCH₃. Where the reactive end groups,X, of the dimethylsilicone fluid are a mixture of --CH═CH₂ and --H, andin near equimolar amounts, the slurry is treated with a platinumcatalyst to set the slurry to rubbery polymer form. No volatiles aregenerated in this process. Where a catalyst is used to set the slurry,it may be added to the slurry immediately prior to application, forconvenience. In addition, heat may be used to aid in the application ofthe slurry to the substrate.

Where the reactive end groups, X, are --OCOCH₃ or --OCH₃, the slurry istreated with water in the form of moist air to set the slurry to rubberypolymer form. The moisture in the air causes a loss of acetic acid ormethanol, respectively, resulting molecular chain growth and therebysetting the silicone precursor.

After setting, the suspended polyimide powder is trapped in the rubberymembrane and laminated to the cloth or substrate. The suspended particlehelp create pathways for water vapor molecules to exit through themembrane. An additional advantage of the silicon rubber is that theinteratomic distance between the silicon and oxygen atoms is such thatenergetic water molecules (water vapor) can readily pass through. Thisstands in contrast to carbon-based rubber.

A fire-resistant, inherently thermally stable, linear aromatic polyimidepowder, like LARC™-IA, when suspended in a rubbery silicone membrane,serves to significantly reduce the overall combustion potential of thecloth or substrate onto which it has been applied. In addition, thepotential for generating toxic combustion by-products when exposed to aflame or high heat source is significantly reduced due to the inherentlynon-flammable nature of the polyimide/silicone mixture.

EXAMPLES Example 1

The following description details the preparation of a fire-resistant,moisture-proof membrane to a substrate for use in WPB fabrics.

A silicone polymer mixture was prepared by blending a silicone, DowCorning Fabric Coating #61 ("#61") having a viscosity of 500 cp, withpowdered LARC-IA™, in proportions of 70% and 30% by weight respectively.A swatch of fabric was pretreated by spray-coating a thin layer of #61on the fabric. The mixture was applied to the pre-treated fabric using acontact printing method. A 40 mesh nylon screen was employed. Severalpasses using a squeegee, 14"/60 durometer, were made to yield asubstantial membrane.

Example 2

The following description details the preparation of a fire-resistant,moisture-proof membrane to a substrate for use in WPB fabrics.

A silicone polymer mixture was prepared by blending a silicone, DowCorning Fabric Coating #61 ("#61") having a viscosity of 500 cp, withpowdered LARC-IA™, in proportions of 80% and 20% by weight respectively.A swatch of fabric was pretreated by spray-coating a thin layer of #61on the fabric. The silicone rubber was set with recommended catalystfrom Dow Corning. The mixture was applied to the pre-treated fabricusing a an off-contact printing method. A 40 mesh nylon screen wasemployed. One pass with the squeegee was made to yield a substantialmembrane.

Example 3

The following description details the preparation of a fire-resistant,moisture-proof membrane to a substrate for use in WPB fabrics.

A silicone polymer mixture was prepared by blending a silicone, DowCorning Fabric Coating #60 ("#60") having a viscosity of 40,000 cp, withpowdered LARC-IA™, in proportions of 80% and 20% by weight respectively.A swatch of fabric was pretreated by spray-coating a thin layer of #61on the fabric. The silicone rubber was set with recommended catalystfrom Dow Corning. The mixture was applied to the pre-treated fabricusing a an off-contact printing method. A 40 mesh nylon screen wasemployed. One pass with the squeegee was made to yield a substantialmembrane.

Further beneficial results are achieved where the coated fabric issandwiched between plastic sheeting and pressure applied, e.g. via ahydraulic press, while the silicone rubber cures. Upon curing, theplastic sheeting is removed to yield the membrane securely adhered tothe underlying fabric.

The embodiments discussed herein were designed and/or tested usingstandard means.

The present invention has been described in detail with respect tocertain preferred embodiments thereof. However, as is understood bythose skilled in the art, variations and modifications in this detailcan be made without any departure from the spirit and scope of thepresent invention as defined in the hereto-appended claims.

What is claimed is:
 1. A process of producing a waterproof, breatheable, fire-resistant laminate by coating a substrate with a membrane, comprising the following sequence of steps:(a) blending a silicone rubber precursor and a polyimide powder into a slurry, the polyimide powder comprised of a fire-resistant, inherently thermally-stable polyimide, where the polyimide powder is comprised of a plurality of particles; (b) applying the slurry to a substrate so as to form a thin layer of slurry on and in the substrate; and (c) setting the slurry layer by inducing the polymerization of the silicone rubber precursor to form a membrane of silicone rubber, so that the silicone rubber membrane is continuous and is connected with and adheres to the substrate, so that the particles of polyimide powder are suspended within the silicone rubber membrane, so that the suspended particles create or help create multiple pathways through the silicone rubber membrane, where the pathways are large enough for a water vapor molecule to travel through but are not so large as to allow liquid water to travel through, so that the silicone rubber membrane repels liquid water but allows water vapor to pass through, and so that the silicone rubber membrane with suspended polyimide powder is resistant to burning.
 2. The process of claim 1, wherein the silicone rubber precursor is a dimethylsilicone fluid with reactive end groups as shown below:

    X--Si(CH.sub.3).sub.2 -- OSi(CH.sub.3).sub.2 !.sub.m --X

where m=5-1000, and where X is a reactive end group selected from the group consisting of a mixture of --CH═CH₂ and --H, --OCOCH₃, or --OCH₃.
 3. The process of claim 2, wherein the reactive end groups, X, are an equimolar mixture of --CH═CH₂ and --H.
 4. The process of claim 3, wherein the setting of the silicone rubber precursor is accomplished by treating the slurry layer with a platinum catalyst.
 5. The process of claim 2, wherein the reactive end groups, X, are --OCOCH₃.
 6. The process of claim 5, wherein the setting of the silicone rubber precursor is accomplished by treating the slurry layer with water in the form of moist air.
 7. The process of claim 2, wherein the reactive end groups, X, are --OCH₃.
 8. The process of claim 7, wherein the setting of the silicone rubber precursor is accomplished by treating the slurry layer with water in the form of moist air.
 9. The process of claim 1, wherein the polyimide powder is comprised of a fire-resistant, inherently thermally-stable, linear aromatic polyimide powder.
 10. The process of claim 9, wherein the polyimide powder has the chemical structure represented below: ##STR2## where n=10-100.
 11. The process of claim 10, wherein the particles of polyimide powder are round and are approximately between 1-20 μm in diameter.
 12. The process of claim 1, wherein the substrate is a substrate selected from the group consisting of cotton cloth and nylon taffeta.
 13. The process of claim 1, wherein the layer of slurry has a thickness of approximately 0.0002 and 0.002 inches. 